Particles of non-oxide materials are currently used in the manufacture of a wide variety of products. Their demand in applications ranging from biomedical implants to aircraft components to electronic components has caused the advanced materials industry to grow enormously over the past several decades. Powders of non-oxide materials such as ceramics, metals, alloys, intermetallics, and metal matrix composites are among the key materials supplied to the industry that allow production of ‘next-generation’ products.
The unique properties of non-oxide ceramics make their potential uses endless. For example, aluminum nitride has high thermal conductivity, yet is a strong dielectric, making it an ideal material for the electronics industry. Production of ceramic parts like aluminum nitride thermal substrates requires powder processing technologies, as ceramics cannot be economically processed by other methods due to their high melting points and poor machinability. In the past, production and manufacturing challenges of ceramics have limited the number of applications. However, recently, structural ceramics, primarily silicon nitride and silicon carbide materials, have evolved into true engineering materials. Over a million highly reliable ceramic turbocharger rotors are currently on the road. Other examples include Cummins' ceramic diesel fuel injector link that has been in use since 1989 and Allied Signal's main shaft ceramic seal runner for the airborne 731 gas turbine engine. The foundation for ceramics to be used as widespread engineering materials has been set by the demonstrated commercial reliability and durability of these, and other, static and rotating structures. Furthermore, their unique properties will ensure rapid market growth in today's competitive materials market.
In the production of metal-based products, powder processing routes may be a requirement to provide a practical or economical advantage by greatly reducing processing times and cost. For example, many of today's automotive gears are made from powder techniques because machining techniques, even today's automated machines, consume a lot of time and material during the fabrication of intricate shapes. Using powder methods, the fabricator needs to form only one intricate shape (the mold) to produce thousands of gears, which contain only slight amounts of excess material, if any. Other examples of metal-based applications that use powders for starting materials during fabrication include iron and ferric alloy (e.g., stainless steel) powders for the manufacture of bearings in the automotive and aircraft industry and titanium powders for use in the production of numerous complex aircraft components. Other applications use material in powder-form in the final product. Examples include platinum powders in automobile catalytic converters, copper powder in anti-fouling paints for boat hulls and in metallic pigmented inks for packaging and printing, and tantalum powders in electronic capacitors.
Low oxygen tantalum powder is one exemplary non-oxide material that is currently experiencing extremely high demand. This is primarily because of one key application for the material: the starting material for production of high quality capacitors. The largest use of tantalum has been in the capacitor market, accounting for 45% of the tantalum in 1986 and growing to over 60% by the end of the 1990s. This is primarily because of tantalum oxide's high dielectric constant and good thermal stability. Tantalum capacitors have been a major contributor to the miniaturization of electronics such as cell phones and palm-top computers.
The heart of these high quality capacitors are anodes consisting of porous bodies of tantalum fabricated by compressing agglomerated tantalum powder to approximately half the full density and then sintering the compressed tantalum powder (with an attached lead wire). The electrodes are then anodized in an electrolytic solution to form a continuous dielectric oxide film on the sintered body. To provide electrical access to the entire free surface of the electrode body, a cathode material is infused into the porous body and a lead wire is connected. The entire device is then encapsulated in a protective, non-conductive material.
The electrical properties of the resulting tantalum capacitor are highly dependent on the characteristics of the starting tantalum powder. Powder characteristics such as average particle size, particle size distribution, particle shape, state of agglomeration, impurity levels and distribution will, in part, determine the charge capacity and the quality of the final capacitor. For example, insufficient interconnection among the tantalum primary particles (i.e. improper agglomerate structure) will lead to the formation of conduction barriers within the anode, which will greatly reduce capacitor performance. Furthermore, the operating voltage and long-term reliability of the capacitor are strongly dependent on the level of impurities in the tantalum powder, the distribution of impurities within the particle (e.g., surface contamination), and the quality of the dielectric film that forms on the surface of the tantalum particles.
The thickness of the dielectric oxide film and the usable surface area of the finished tantalum anode primarily establish the capacitance of the final device. The capacitance is a measure of how well the capacitor can accept charge. Capacitance is directly proportional to surface area and inversely proportional to the thickness of the dielectric film, and, as a result, capacitors produced from smaller primary particles use lesser amounts of tantalum powders. This increased capacitance per unit mass allows designers in the consumer electronics industry to reduce the size of their product or maintain an existing size and add performance capabilities. The drive for smaller components coupled with the ability to increase volumetric efficiency, measured by the product of capacitance and voltage (CV), has resulted in considerable commercial effort to decrease the size of tantalum powders. Modern high-CV powders have Fisher Sub Sieve Sizes of less than 1-2 micrometers. From such powders, capacitor manufacturers have succeeded in producing powders with volumetric efficiencies in the range of 70-80 millifarad-volts/gram.
However, most production methods used today to produce capacitor powders are extensions of the processes that were first developed decades ago and, as such, are not ideally suited to produce the high surface area powders required today. Improvements have been made over the years, but the production methods are inherently limited. Current tantalum production methods include two primary types: mechanical or chemical. Although these conventional methods of processing tantalum have had some success at decreasing the size of the powders, many challenges remain before they are capable of producing ultrafine tantalum suitable for capacitors.
One critical challenge is controlling the level of impurities in the high surface area (high CV) powders. The purity of the material is critical, as the quality of the dielectric layer that is formed on the surface of the sintered powder is very sensitive to the purity of the base metal. Purity is less of a problem for low CV powders that are sintered at temperatures near 2000° C. because substantial purification can occur, as many of the impurities are volatile at such temperatures. High CV powders must be sintered at lower temperatures to minimize the coarsening of the particles. Consequently, this decrease in processing temperature greatly reduces sintering purification and, thus, places a higher demand on the purity of the starting powders. In addition, typical high surface area tantalum powders suffer from excessive oxygen contamination because the tantalum has a very high affinity for oxygen and, as particle size decreases, the surface area for a given mass increases. What was once an insignificant surface layer of tantalum oxide now can represent a significant fraction of the total weight of the powder.
Tantalum powder produced via the conventional chemical route (e.g., liquid phase sodium reduction of potassium fluorotantalate) results in the tantalum powder having a high surface area, but suffering from low purity. The conventional mechanical process, electron beam melting, results in tantalum powder having a higher purity, but suffers from low surface area. Generally, making a capacitor-grade powder from these processes requires numerous steps after reduction of the tantalum precursor to tantalum metal. The additional steps focus on converting the raw tantalum powder into a powder with well-defined characteristics.
One promising route to produce submicron powders of non-oxide materials such as tantalum is through aerosol gas-to-particle processing. Over the last three decades, the understanding of the physico-chemical processes occurring in gas-to-particle conversion routes has advanced significantly. Gas-to-particle conversion routes have been used to produce particles in a broad range of sizes, from nanometer to submicrometer scales, with size distributions from nearly monodisperse to polydisperse. With the increased interest in production of nanophase powders, over twenty different gas-to-particle processes have been developed to address this need. These include furnace reactors, gas condensation techniques, sputtering, plasma reactors, laser ablation and flame reactors.
Typical gas-to-particle conversion processes produce a condensable vapor of the desired material through gas-phase reaction or vaporization/sublimation. Then, depending on the conditions within the reactor, nucleation, condensation, evaporation and surface reaction can occur as the molecules grow to form particles. While the particles are small, the high surface energies result in the formation of spherical particles. However, as the particles grow in size, the time for particles to fully sinter (coalesce) into spherical particles increases. When the sintering time becomes longer than the time between collisions, the particles are unable to fuse into single spherical particles. Limited sintering results in the formation of agglomerated particles. If no controls are in place to shape how the particles come together, long chain agglomerates may form. FIG. 1 shows how these long chains of agglomerated particles are formed in the conventional flame synthesis process. These long agglomerated particle chains are undesirable because they are difficult to compress into a dense, tightly-packed powder. An appropriate analogy would be the difficulty that one would experience when trying to compress branched tree limbs into a dense, tightly-packed mass of wood—it would be considerably easier to use saw dust as the starting material.
The common approach to minimize agglomeration in gas-to-particle aerosol routes has been to decrease the particle number density. By doing so, collision frequencies among particles are decreased, which thereby reduces the extent of agglomeration. However, a decrease in the particle number density often results in a lower production rate. Accordingly, this approach is undesirable because it inherently limits the scale of the process to production of only modest quantities of powders.
Another approach to minimizing particle agglomeration is disclosed in the inventors' prior U.S. Pat. No. 5,498,446 (the entire disclosure of which is hereby incorporated by reference) which discusses the production of high purity, unagglomerated nanopowders of metals and non-oxide ceramic materials. The '446 patent discloses a technique which can be referred to as the SFE process (sodium/halide flame encapsulation). The SFE process encompasses the reaction of a metal halide with an alkali or alkaline earth metal to yield two condensable products. An example of the chemistry employed for the production of titanium (Ti) by the SFE process of the '446 patent is as follows:TiCl4+4Na+Inert→Ti+4NaCl+Inert
According to the '446 patent, if the NaCl is initially in the vapor phase, the early stages of the process are similar to the standard flame process: reaction followed by nucleation and growth of the aerosol. However, before conditions that favor the formation of long-chain agglomerates are reached, the particles are encapsulated by triggering condensation of the second component. The second component can be independently added or can be a byproduct of the reaction forming the primary particles. Through the addition of a second condensable phase to the process, the primary particles can be encapsulated in-situ. Provided the encapsulate does not absorb moisture or oxygen, it can protect the high surface area primary particles from oxidation and/or hydrolysis, thereby preserving the purity of the particles. The ability to encapsulate highly reactive particles in-situ represents a significant improvement over conventional methods of preventing particle contamination. FIG. 2 shows a typical transmission electron micrograph (TEM) of titanium particles produced using the SFE technique of the '446 patent. The image shows a dark particle (titanium particle) within a lighter material (the sodium chloride encapsulate). Clearly the titanium particles are not in contact with other titanium particles, and are therefore unagglomerated.